FIG. 10 is a block diagram showing the pertinent portion of the numerical control system in the prior art. In FIG. 10, 1 is a CPU, 2 is a memory, 3 is an external input/output portion, 6 is a set/display controlling portion, 9 is a calculating/analyzing portion, 10 is an interpolation processing portion, 11 is an acceleration/deceleration processing portion, 12 is a driving unit 1/F portion, 15 is a PLC processing portion, and 21, 22 are servo amplifiers that drive/control servo motors 31, 32 to positions in compliance with the command pulse generated by the NC system and also move the axes to commanded positions.
As is well known, the NC system analyzes the NC program loaded in the memory 2 or the like, then generates moving command pulses indicating the command positions on respective axes, and then drives the tool, etc. Normally, the command pulse being issued to the servo amplifiers has the unit of a physical length, for example, one pulse has 0.1 μm. With regard to the feed on the NC program being commanded on the orthogonal coordinate system, the interpolation pulses are generated to the driving units, which feed the shafts in commanded axis directions, in response to an amount of feed respectively.
Also, as disclosed in Patent Application Publication (KOKAI) Hei 5-341823, there is the case that, for the purpose of size reduction of the machine, etc., in some case the feed shaft for driving the tool slide, or the like is arranged to be inclined by an angle other than 90°. Such an oblique shaft control technology is present that command pulse is output in response to the NC program, which issues the command on the orthogonal coordinate system, in terms of an amount of feed in the shaft direction that is obliquely arranged and then the tool slide, or the like is moved in the commanded axis direction on the orthogonal coordinate system that is commanded by the NC program.
FIG. 11 is an example of the oblique shaft control. The NC program issues the command on the orthogonal coordinate system Xp-Zp, but the moving directions of the actually provided shafts are the Xm-axis direction and the Zm-axis direction. The Zm-axis direction is arranged at an inclination to the Zp-axis direction by an angle θ. If the moving command from Za to Zb is issued by the program command, the NC system generates the command pulse indicating the amount of movement Lz to the Zm-axis servo control portion. At this time, since the tool position is displaced in the Xp-axis direction, the command pulse indicating the amount of movement of the length Lx is generated to the Xm-axis servo control portion at the same time to correct the displacement of the nose in the Xp-axis direction. The moving command pulses at this time are calculated byLz=(Zb−Za)*(1/cos θ) Lx=Lz*sin θ. 
In the above NC system in the prior art, in the case that the feed shaft direction along which the command pulse is actually output is different from the command axis direction that is commanded by the NC program on the orthogonal coordinate system, for example, in the example shown in FIG. 11, the actual feed shaft (Zm axis) direction is inclined from the feed in the Zp-axis direction by an angle θ with respect to the command on the orthogonal coordinate system Xp-Zp. Therefore, the actual feed is given as a synthesized feed of the movement in the Xm-axis direction and the movement in the Zm-axis direction in response to the command in the Zp-axis direction.
Accordingly, in the case that the conventional NC system controls the machine tool, etc. having the feed shafts such that the Zm axis is inclined to the Xm axis by an angle θ, it is impossible to issue the moving command on the Zm axis (the feed shaft that is inclined to the Xm axis by the angle θ) only in response to the feed command on the orthogonal coordinate system.
That is, there is such a problem that only the Zm axis cannot be operated not to cause the operation of the Xm axis.
In this connection, in the case of the above machine, if only the Zm axis cannot be operated without the operation of the Xm axis, it is impossible to carry out the process during which a movement of the feed shaft must be controlled in synchronism with the rotation of the spindle), etc., for example, the tapping process in the Zm-axis direction (this process is carried out by fitting the tapping tool to the rotated spindle and then feeding this spindle in the Zm axis direction in synchronism with the rotation of the spindle).
In light of the above disadvantageous point, it may be considered theoretically that the program orthogonal coordinate system should be set to not the orthogonal coordinate system but the coordinate system whose axes coincide with the Zm axis and the Xm axis. However, normally the machining plan is set forth by the third angle projection method. Therefore, as described above, if the program orthogonal coordinate system is set to not the orthogonal coordinate system but the coordinate system whose axes totally coincide with the feed shaft directions of the machine, there is often caused the case that the data set forth in the machining plan cannot be employed as they are in preparing the program. Thus, there are disadvantages that the command values must be calculated based on the data set forth in the machining plan, and thus such coordinate system is not practical.